The present invention relates to a cascade arc plasma torch for use in low temperature plasma polymerization coating. The invention also extends to a process for plasma polymerization at low pressure and at low temperature.
Plasma polymerization is a technique which emanates from the early 1960's. In the early stage of development of this technique it was a widely held impression that plasma polymerization was a highly exotic method used to form an ultra-thin layer of a polymer. The method was of interest mainly as a curiosity in the methodology of polymerization, since the resulting plasma polymer was believed to be identical to a conventional polymer derived from the same monomer. During the more than two and a half decades that have elapsed since the earliest development took place it has become apparent that polymers produced by this technique have characteristic properties differing from those of polymers produced by more conventional means. Today it is more appropriate to describe plasma polymerization as a unique method which can be used to prepare special materials.
Today plasma polymerization is a technique which has left the stage of laboratory curiosity. Although it is difficult to estimate the extent to which highly proprietary industrial operations are presently involved in plasma polymerization activities, it is known that some large scale industrial applications of plasma vacuum deposition polymerization are currently used. As examples there may be mentioned that the head light reflectors used in most European cars are manufactured by Bosch using a large scale vacuum deposition process. In this practical application the vacuum deposition of metal for the reflecting surface and the deposition of plasma polymer onto the metal surface to protect it from corrosion and tarnishing are combined into one continuous process. 3M Company produce optical storage discs by vacuum deposition of various components including methyl methacrylate. The indicated industrial processes use plasma polymerization to an extent which verifies that it has achieved an importance far beyond that of a mere laboratory curiosity.
However, the techniques presently used for deposition by a plasma polymerization are associated with severe drawbacks among which the following may be mentioned.
In conventional plasma polymerization the total substrate surface area that can be coated evenly is limited by the total volume of plasma because the substrate surface must be immersed in the plasma volume.
The yield of deposited polymer in relation to the monomer used is very low in conventional plasma polymerization due to the fact that the plasma volume accommodating the substrate contains such monomer throughout its whole volume.
Excessive fragmentation of monomer molecules takes place due to ionization resulting in even more impaired yield of deposited polymer.
The present invention has for a main object to provide new techniques based on the use of a cascade arc plasma torch operated at low temperature for plasma polymerization coating while eliminating or at least greatly reducing the drawbacks associated with the prior art plasma polymerization techniques.
Another object of the invention is to provide a cascade arc plasma torch for use in low temperature plasma polymerization.
Yet another object of the invention is to provide a process for plasma polymerization at low pressure and at low temperature using a cascade arc generator.
The present invention is based on an entirely new concept whereby a monomer or a mixture of monomers will be injected into a plasma torch or plasma jet and not into a vacuum chamber as in the conventional art. Due to the high velocity of gas in the low temperature plasma torch generated in a cascade arc generator the back diffusion of monomer molecules to the energy input zone where ionization occurs is practically nil. This means that the monomer introduced into the cascade arc torch will not be subjected to the ionization process, which by some investigators is believed to be an essential step for plasma polymerization. Although the invention is not to be construed to be limited to any specific mechanism or operational principle it has been found quite surprisingly that plasma polymerization takes place without the ionization of monomers which up to now has been believed to be necessary.
Accordingly, the present invention provides for a cascade arc plasma torch apparatus for use in low temperature plasma polymerization coating, said apparatus comprising means for vacuum generation, a plasma reactor including concentric electroconductive rings separated by insulator rings, said rings being arranged between electrode connectors and forming a central passage through said plasma reactor, a voltage supply source providing a voltage across said plasma reactor, and supply means for introducing an inert gas into said passage. Such apparatus is characterized by first inlet means positioned at the downstream end of the plasma reactor for feeding monomeric gas enabling plasma polymerization into said passage at the downstream end thereof.
Such first inlet means for the supply of monomeric gas is preferably constituted by inlet parts or nozzles distributed around the periphery of the central passage of the apparatus.
According to another aspect of the invention the apparatus can be provided with powder inlet means positioned adjacent to said first inlet means, preferably downstream thereof, which enable introduction of powder into the generated plasma for plasma polymerization processing of powder. Such powder inlet means can likewise be constituted by inlet parts or nozzles distributed around the periphery of the central passage.
Furthermore, the invention provides a process for plasma polymerization at low pressure and at low temperature, said process comprising the steps:
(a) creating a plasma in a cascade arc generator to form a plasma torch which is directed into a low pressure zone; PA1 (b) injecting a monomeric gas into said plasma torch; and PA1 (c) directing the plasma torch resulting from step (b) onto a substrate to form a film thereon by plasma polymerization.
The pressure of the low pressure zone is preferably less than about 100 Torr. The monomeric gas supplied into the plasma zone preferably contains monomers selected from hydrocarbons, halogenated hydrocarbons, silanes and organosilanes optionally together with hydrogen.
According to a special feature of the invention the substrate to be coated by plasma polymerization is a powder, such as a metallurgical powder, which is introduced into the plasma torch downstream of the site of injection of the monomeric gas.
According to yet another aspect of the process of the invention the substrate is an elongated member such as a wire, a tubing, a band or a filament, etc., introduced in the direction of its length into the process before the monomeric gas injection site, whereby the pretreatment of substrate surface and a deposition of polymer could be achieved in a single uniform process. By such pretreatment undesirable contamination on the substrate can be easily removed.
The present invention provides new techniques, whereby a high yield of deposited polymer is obtained and a well defined localized polymer deposition is made possible. In order to obtain uniform coating of a wide area the plasma torch can be arranged to scan the location of deposition in a predetermined pattern. An important aspect in this context is the fact that the invention provides freedom from the limitation based on plasma volume/surface area which is encountered in the conventional plasma polymerization using glow discharge.
Another advantage of the low temperature plasma torch technique described herein is the fact that the coating operation is less disturbed by other external factors, such as magnetic field and the presence of Faraday Cage effect and, consequently, the process of the invention can be applied to the coating of the inner surface of a Faraday Cage, such as the inner surface of an automobile body.
The present invention also enables powder processing by a plasma polymerization. The main object of this aspect of the invention is to alter the surface characteristics of powder by applying an ultra-thin layer of plasma polymer coating. Such applications include sintering powders in general, such as ceramic powders and metal powders for powder metallurgy. Furthermore, the invention is useful for protective coating of metallic pigments, such as aluminum powder used in automotive paints, and coloured pigments and fillers used in paints and plastics. In this context other gases, such as oxygen, for example for anodization of aluminum powder, and methane can be fed through the arc system by mixing with an inert gas, such as argon. More complex monomers could be fed through the monomer inlet located downstream of the central passage of the plasma torch apparatus.
By injecting a monomeric gas into the plasma torch created by the cascade arc at the downstream end of the plasma reactor passage chemical reaction of the monomers with the excited species in the plasma can be utilized to perform polymeric deposition without subjecting the monomer molecules to ionization. This is an advantage since the ionization of organic molecules causes extensive fragmentation of the original molecular structure thus reducing the yield of deposited polymer and also altering its properties.
In most plasma polymerization procedures using glow discharge an energy input of 0.1 to 15 GJ/kg monomer is generally found to be necessary to form plasma polymers. However, this high energy level also causes excessive fragmentation of the original monomer structure. Using the technique of the present invention a low pressure cascade arc can be generated at an energy input of 1 to 10 MJ/kg monomer. This is several orders of magnitude lower in energy input than that used in conventional art. Therefore, the low pressure cascade arc of this invention offers the possibility of forming polymeric films whose chemical structure can be controlled primarily by the selection of monomer structure and only secondarily by energy input level.
In a cascade arc reactor of this invention the monomeric gas injected into the reactor cannot diffuse back into the cascade arc generator because of the high forward flux of the plasma. This results in a surprisingly high deposition rate of polymer despite the relatively small monomer flow rate compared to that of the inert gas, such as argon, entering the reaction chamber. This means that the rate of the chemical reaction in such system is high and that most of the monomer or mixture of monomers injected into the reactor are quickly and efficiently consumed by the flame and deposited on the substrate.
The invention will now be further illustrated by non-limiting embodiments and specific examples. The embodiments will be described in conjunction with the appended drawing, wherein: